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## Chapter 13. Playing God: Basic Physics Modeling

Morpheus, The Matrix

There wasn't a whole lot of physics involved in the video games in the '70s and '80s. For the most part, games were shoot-'em-ups, search-and-destroy games, adventure games, and so on. However, beginning with the '90s and the "3D era," physics modeling became much more important. You simply can't get away with having objects in games move in non-realistic ways—the motion of the objects has to at least approximate what you'd expect in the real world. This chapter covers fundamental non-calculus-based physics modeling. Then, in the second volume I'll cover more rigid, calculus-based 2D and 3D modeling. Here are the topics you'll see in this chapter:

• Fundamental laws of physics

• Gravity

• Friction

• Collision response

• Forward kinematics

• Particle systems

• Playing God

If the universe is just a simulation in some unbelievably advanced computer, God is one heck of a programmer! The point is, the laws of physics work perfectly at all levels, from the quantum level to the cosmological level. The beauty of physics is that there aren't that many laws that govern the whole universe. Granted, our knowledge of physics and mathematics is that of a Cabbage Patch doll, but we do know enough to create computer simulations that can fool just about anyone.

Most computer simulations and games that use physics models use models based on standard Newtonian physics—a class of physics that works reasonably well on motion and objects that are within reasonable limits of size and mass. That is, speeds much less than the speed of light, and objects that are much bigger than a single atom, but much smaller than a galaxy. However, even modeling reality with basic Newtonian-based physics can take a lot of computing power. A simple simulation like rainfall or a pool table (if done correctly) would bring a Pentium IV to its knees.

Nonetheless, we have all seen rainfall and pool games on everything from the Apple II to the PC. So how did they do it? The programmers of these games understand the physics that they're trying to model and within the budget of the system they're programming on, create models that are close enough to what the player would expect in real life. This programming consists of a lot of tricks, optimizations, and most of all assumptions and simplifications about the systems that are being modeled. For example, it's a lot easier figuring out how two spheres will react after collision than it is to calculate the result of the collision of two irregular asteroids, thus a programmer might approximate all the asteroids in a game with simple spheres (as far as the physics calculations go).

In a state-of-the-art game, physics would take about 10,000 pages because it's not only the physics, but the math that needs to be learned, so I'm just going to cover some of the most fundamental physics models. From them you should be able to model everything you need in your first 2D/3D games. Most of this stuff should be more than familiar from High School physics—or Junior High School!

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